The present invention relates to a method, a circuit arrangement and an apparatus for a non-contacting real time determination of velocities of one or a plurality of movements at an object in a direction perpendicular to a coherent radiation impinging on the object, with the scattered light of the coherent radiation producing a spatial speckle pattern over a speckle spectrum, and with the speckle pattern becoming time dependent due to the movements at the object.
If a laser beam of wavelength .lambda. impinges on an object in the x direction, and the average distance .DELTA.x between scatter centers is &gt;&gt;.lambda., the scattered light forms a granular structure, called speckles. This is the case, for example, on all normal surfaces. The intensity distribution of the speckles is irregular. Certain mathematical interrelationships exist for the statistic distribution of the speckles. These mathematical interrelationships do not depend on the characteristics of the object, as long as the requirement of .DELTA.x&gt;&gt;.lambda. is met.
With the laser at rest and the object at rest, the speckle pattern is stationary. Under certain conditions of mutual position of laser, object and observation point, the speckle pattern moves as a whole if the object itself moves. This results in a specific speckle velocity V.sub.s which, in suitable cases, is proportional to the velocity of the object. One way to record speckle movement is to measure the light intensity of the scattered light on an effective detector surface which is small compared to the average speckle size .sigma..
Such a detector produces a signal I proportional to the light intensity on its surface and there now exist various ways to determine the speckle velocity from the time-dependent form, I(t), of that signal. One of these methods is the so-called correlation method. Here, the incident light intensities are detected at two spatially separate locations and the resulting intensity signals are recorded. Two time dependencies result for the intensity. Both intensity time dependencies are stored. The correlation function calculated therefrom has a peak from whose position the speckle velocity can be determined.
The drawback of this method is that it requires a large amount of electronic equipment and that the velocity is not determined in real time (Pusey, J. Phys. D 9 (1976) page 1399). In principle, this method does not utilize any specific speckle characteristic.
Only a single time dependent signal I(t) is required for the method employing time integration of the signal I(t). Initially, average values N.sub.i are formed from the speckle intensity signal I(t) by way of integration over a succession of time intervals, with a computer calculating the standard deviation S and an overall average N. The quotient of both values, S/N, is a non-linear measure for the speckle velocity. The drawback of this method is again the high costs for the electronic computer and the fact that it is not a real-time method. In principle, the fact that the contrast of the speckle signal is known to go toward zero with increasing integration time, is utilized here (J. Ohtsubo, T. Asakura, Opt. Quant. Electr. 8 (1976) 523).
An earlier patent application, in the Federal Republic of Germany, No. P 3,242,771.9, discloses the so-called speckle counting process. This method counts the points of intersection between the intensity signal I(t) and a threshold level S. The threshold level S is set to be proportional to the average intensity value I. The advantage of this method is that the costs for electronic equipment are reduced and the method has real-time characteristics.
However, the drawback of this method is that noise is superposed on the speckle signal. Particularly at low velocities, this noise results in erroneous measurements because the velocity indicated is too high. In principle, this method utilizes the speckle characteristic that the time interval .DELTA.t between two counts has an average .DELTA.t which is given by .DELTA.t being approximately const.multidot..sigma./V. None of the prior art methods is suitable to separate superposed velocities.